Light as a feather, stiff as a board

Nanotubes have been held out as the technology of the future. Their time may have come: scientists have made a semiconductor device out of them

SEMICONDUCTOR technology today relies on silicon, the second most abundant element on earth. But after only 50 years of silicon-based computer chips, physicists are already thinking about its limitations, and have been searching for a suitable successor. Materials scientists, meanwhile, dream of creating an amazingly strong, light, flexible substance that could be used in everything from car bumpers to tennis racquets. Both groups think they may have found one. It is in soot.

Soot is made of carbon. And carbon has some unique properties that scientists are still learning about. In 1985 Richard Smalley, Robert Curl and Harold Kroto discovered that, when carbon is vaporised in an inert gas such as helium and allowed to cool slowly, it spontaneously forms a buckyball, the celebrated soccerball-shaped carbon molecule consisting of 60 carbon atoms arranged in a symmetric pattern. Buckyballs are unusual structures: chemically inert, incredibly strong and in some cases electrically and thermally conductive. But despite the thousands of research papers devoted to buckyballs, practical applications remain elusive. As a result, attention has been shifting to “buckytubes” or “nanotubes” as structures that could work technological wonders.

Buckytubes belong to the same carbon family as buckyballs. They are made by adding millions of extra sets of hexagons of carbon atoms to the middle of the soccerball molecule so that it stretches out to form a tubular fibre. For industrial purposes, the tubes have always looked more promising than their spherical cousins. Indeed, in the six years since nanotubes were first fabricated, researchers have been stunned by the electronic properties they can have. The latest surprise is that nanotubes can function as semiconductor devices.

A contortionist's fantasy

With his colleagues, Alex Zettl, a physicist at the University of California, Berkeley, was using an instrument known as a scanning-tunnelling microscope (STM) to peek into the microscopic world of the nanotube, which is only tens of atoms in diameter. An STM is most famous for the dramatic atomic-resolution images of conducting surfaces made by scanning a sharp tip over the region of interest and controlling it so that it can sense even the bumps of individual atoms. But an STM is also routinely used to measure a local current in a material, giving the experimenter an idea of the electronic properties at a precise spot.

Dr Zettl and his colleagues used an STM to isolate a single nanotube, about 20 nanometres (millionths of a millimetre) in diameter. To do this, they rammed the tip of the STM on to a substrate full of nanotubes and then slowly withdrew the tip. Like a sticky thread of cotton candy, a single tube unravelled itself from the mess, one end sticking to the tip and one end remaining in contact with the substrate. The researchers could then guide the tip across the length of the tube, measuring the current at intervals. In an article in Science last week, they reported that certain parts of the tube behaved like a metal: a current varied smoothly as the voltage applied to it was changed. Elsewhere, the current came on only after the voltage reached a particular threshold, much like a semiconductor.

This discovery is highly significant. Dr Zettl's nanotubes provide, within the space of tens of atoms, an almost perfect metal-semiconductor junction. Engineers already have a name for such junctions: they are called Schottky barriers and are usually made by forcing a metal such as gold to stick to a semiconductor such as silicon. But making a conventional Schottky device this small is hard. The atoms invariably spread out and end up in places where they are not wanted, causing the device to degrade. The devices also tend to heat up because they do not conduct heat especially well. Carbon-nanotube devices have neither drawback because they are all one material and are excellent thermal conductors.

A separate breakthrough, reported in this week's Nature, has also added to the nanotube's lustre. They were always renowned for their strength. But new research, led by Richard Superfine, a physicist at the University of North Carolina at Chapel Hill, suggests that they are almost impossible to break.

Unlike the single-walled tubes that Dr Zettl used to make the electrical measurements, Dr Superfine and his colleagues worked with multiwalled nanotubes, so called because they consist of several tubes nestled one inside the other like a Russian doll. They inserted each of their multiwalled tubes into the “nanomanipulator”. This is a specially designed atomic-force microscope (a close relation of the STM), which uses the forces between atoms to move the tip. The nanomanipulator bent, buckled and squeezed each tube into the most strenuous contortions. Yet the tubes were generally able to survive unscathed, even under the most severe strain.

For now, producing nanotubes for any particular application remains a haphazard business. But their boosters believe that a world of nanomaterials is approaching fast. Graphite—another form of carbon—is already used for its strength and lightness to reinforce tennis racquets and aircraft parts. Because of their resilience, carbon nanotubes may well work better for such applications. They are already being used as a component of certain plastics that are turned into car bumpers.

Displacing silicon as the staple composite of semiconductor devices will be more difficult. Dr Zettl's team does not yet know how to make a nanotube semiconductor device reliably. At present, a whole mass of nanotubes is produced and the ones that show the right device-like behaviour are picked out individually. This is far too laborious for nanotubes to be used in microelectronic applications right away.

Still, Dr Zettl thinks it can be done soon. He envisions new companies springing up within a year to exploit carbon nanodevices. Apart from attaching wires to them and using them as standard Schottky devices, he foresees the creation of tiny “nanocomputers”. One approach would be to generate a random network of devices made from nanotubes, and then see what sorts of outputs they give for certain inputs. It might be easier to create a machine and then learn what it can do than to design one for specific pre-determined tasks. Dr Zettl and his group are already modelling just such a nanocomputer. Within a decade, he predicts, silicon may face some sooty competition.